AstroSat and MAXI view of Cygnus X-1: Signature of an `extreme' soft nature

Abstract

We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during $2016$ to $2019$, by SXT and LAXPC on-board AstroSat. We model the spectra in broad energy range of $0.5-70.0\mathrm{keV}$ to study the evolution of spectral properties while Cygnus X-1 transited from hard state to an extreme soft state via intermediate states in 2017. Simultaneous timing features are also examined by modelling the power density spectra in $3.0-50.0\mathrm{keV}$ . We find that during high-soft state observations, made by AstroSat on Oct $24,2017$ (MJD $58050$), the energy spectrum of the source exhibits an inner disk temperature (kT${}_{\mathrm{in}}$) of $0.46\pm 0.01\mathrm{keV}$ , a very steep photon index ($\Gamma$) of $3.15\pm 0.03$ along with a fractional disk flux contribution of $\sim 45\%$. The power density spectrum in the range of $0.006-50.0\mathrm{Hz}$ is also very steep with a power-law index of $1.12\pm 0.04$ along with a high RMS value of $\sim 25\%$. Comparing the spectral softness of high-soft state with those of previously reported, we confirm that {\it AstroSat} observed Cygnus X-1 in the `softest' state. The lowest MAXI spectral hardness ratio of $\sim 0.229$ corroborates the softest nature of the source. Moreover, we estimate the spin of the black hole by continuum-fitting method, which indicates that Cygnus X-1 is a maximally rotating `hole'. Further, Monte Carlo (MC) simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a${}_{\ast }>0.9981$ with $3\sigma$ confidence interval. Finally, we discuss the implications of our findings.

13 pages, 8 figures

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